This invention relates to the manufacture of crystalline materials primarily for acoustic and optical applications, and it relates, more particularly, to the formation of homogeneous crystals in an improved process wherein substantially all of the liquid material is efficiently converted into crystalline material of spatially uniform properties throughout its volume.
The useful properties of single crystalline lithium niobate, such as associated with its energy transmissive crystal structure and piezoelectric nature, have long been recognized and are responsible for extensive and widespread applications for sometime. Although often noted as "LiNbO.sub.3 ", the lithium niobate phase encompasses a relatively broad range of composition, with possible Li to Nb elemental ratios from roughly 44 to 50 mol % Li. Due to the rather adaptive nature of the dynamic, complex, crystal growing process for forming such crystalline material, efficiency of production and uniformity of product have until now continued to elude crystal growers.
Lithium niobate is typically prepared in single crystal form by pulling crystals in the well-known Czochralski process. Single crystal boules as large as 10 kg have been prepared in this way.
Central to any solidification process, including the Czochralski process, is the concept of "congruency". A liquid is considered to solidify "congruently" if, on cooling, it produces a solid of exactly the same composition as the liquid. By contrast, in incongruent solidification, the solid forming at any time has a composition different from that of the immediately adjacent freezing liquid. The consequence is rejection of some chemical component from the solidification front back into the liquid. As solidification progresses, this rejection effects a gradual enrichment of the liquid by the rejected species which in turn results in spatial variation of the chemical composition of the resulting solidified body. As a result, the failure to begin with the exact "congruent" composition drives the liquid phase composition even further away from the correct composition which makes it impossible to produce crystals of spatial uniformity in a high yielding process.
In a practical production process for single crystalline material of a given phase, congruent solidification, if possible, is generally preferred to incongruent solidification largely for two reasons. First, from a crystal production standpoint, a congruent solidification process is generally more efficient. Due to the rejection phenomena going on at an incongruent solidification front, conventional growth rates must be kept relatively low to avoid interfacial breakdown effects that would otherwise result in a highly defective product. Also, due to the build-up of rejected species in the liquid as incongruent solidification progresses, the fraction of a given volume of liquid that can be successfully crystallized is limited. In addition, the spatial variation in composition in a crystal grown incongruently may also result in spatial variation of its thermal expansion properties. As a consequence, there is a tendency to crack on subsequent cooling.
Second, from the standpoint of using fabricated crystalline material in construction of electronic, optical, or acoustic devices, a congruent solidification process yields more desirable material. Efficient production of high-performance crystal-based devices to tight design tolerances requires high uniformity in critical crystal material properties, both within individual pieces and from piece to piece. Generally, such uniformity in critical material properties in turn requires uniformity in crystal composition. Clearly, a congruent solidification process favors such uniformity in crystal composition.
The subject of congruency as it relates to lithium niobate Czochralski growth has been a rather confused issue from the earliest work on the material. In large part, difficulty in resolving the confusion has been due to the practical difficulty in measuring to necessary precision the chemical composition of crystalline lithium niobate. Due to the extremely low solubility of solid lithium niobate in common acids, standard wet-chemical analytical techniques are relatively imprecise. Potentially more accurate techniques like Curie temperature measurement and non-linear optical phase-match temperature measurement are rather subtle in their application.
Initial lithium niobate growth work was based on the belief that the congruent composition should be a Li:Nb ratio of 50 mol % Li. In 1968 Lerner et al. in the Journal of Crystal Growth 3/4 (1968) 231 reported the congruent composition to lie between 48 and 49 mol % Li. Later, Carruthers et al. in the Journal of Applied Physics 42 (1971) 1846 reported the congruent composition as 48.60 mol % Li. In 1974, Chow et al., Mat Research Bull. 9 (1974) 1067 determined that the congruent composition lies between 48.50 and 48.60 mol % Li. In 1985, O'Bryan et al. in the Journal of the American Ceramic Society 68 (1985) 493 reported the congruent composition as 48.45 mol % Li. Most recently, in Applied Physics Letters 53 (1988) 260 Wen et al. made an uncited reference to a congruent composition value of 48.71 mol % Li.
Obviously, not all of the foregoing conflicting reports on congruent compositions can be correct. Moreover, each of these studies is open to serious doubt, due to one or more of several shortcomings. Primarily, these drawbacks are:
(1) Inaccuracy or imprecision in compositional measurement technique, PA1 (2) Growth of crystals representing total melt fractions smaller than desirable for maximizing effects of incongruency, PA1 (3) Compositional characterization of specimens inappropriately large with respect to the size of the parent crystals being analyzed, and PA1 (4) Analysis of measurement results based on dubious extension of approximate theoretical construction.
If one were to take as an illustration the recent work by O'Bryan et al., these people skilled in the art based an analysis of a preferred lithium niobate growth composition on compositional measurements they performed on both fabricated crystalline specimens cut from lithium niobate crystals and solidified polycrystalline samples withdrawn from the corresponding melts used in crystal growth. The compositional measurements were performed using a differential thermal analysis (DTA) technique to determine the ferroelectric Curie temperature of each of the samples. It is also significant to note that the fraction of liquid crystallized only corresponded to 0.72 of the melt.
The precision on the Curie temperature measurements of O'Bryan et al. was quoted as .+-.2.degree.C. Such precision in Curie temperature measurement corresponds to a roughly .+-.0.2 mol % Li precision in specifying melt composition for growth. It is generally known and is confirmed in data and discussion presented hereinafter that such wide error bars associated with specifying melt composition are unable to provide a significant improvement in lithium niobate crystal production beyond present conventional commercial practices of those skilled in the art.